Conventional velocity servo systems for controlling a printer carriage utilize an analog tachometer as a feedback device to provide a signal representing the actual carriage speed. This tachometer signal is generally summed with a command signal to produce an error signal used to control the motor driving the carriage. The tachometer is normally designed for a particular relatively narrow speed range and compromises in accuracy and response time must be tolerated when it is desired to broaden the speed range.
A digital servo system for a printer carriage has been proposed intended to avoid the aforementioned performance compromises and is disclosed in a copending commonly assigned application Ser. No. 382,107 filed May 26, 1982 for S. D. Moon.
Printer analog servo systems are also known which attempt to avoid the aforementioned compromises by using specially designed circuits employing carefully matched components. However, such solutions have generally proved to be expensive because of the number of parts and the number of precision adjustments required in the manufacturing of such a printer. Moreover, these specially designed circuits generally occupy more circuit board space than is desirable and their overall complexity represents a reliability concern.
Inexpensive integrated circuit packages, (e.g. the SGS-ATES L290 and L291) are commercially available and useful for carriage speed control in relatively slow speed printers. More specifically, the aforementioned integrated circuit packages, when used in their intended manner, are adequate in a servo system for printers operating at about 15 characters per second. However, when used as intended, these devices do not afford the range and accuracy required for higher speed (e.g., in excess of 30 characters per second) letter quality printers.